This invention relates to lasers. More particularly, it is concerned with aluminophosphate glass compositions containing high concentrations of trivalent neodymium ion, utilizable as active laser media and with lasers employing such media.
There has been considerable recent interest in attempts to produce small lasers which utilize stoichiometric neodymium compounds as the active laser media. These efforts have been reviewed by S. R. Chinn et al. in Laser Focus, pp. 64-69, May, 1976. Single crystals of such phosphate compounds as LiNdP.sub.4 O.sub.12 and NdP.sub.5 O.sub.14 have been tested as laser media. These compounds, known respectively as meta- and ultraphosphates, are examples of stoichiometric neodymium laser materials; that is, compounds in which neodymium forms one constituent of a stoichiometric formulation rather than merely a dopant.
These highly concentrated neodymium-containing laser media permit the construction of miniature lasers because of the large number of laser active sites per unit volume of the crystal. The neodymium content in NdP.sub.5 O.sub.14, for example, is 4.0.times.10.sup.21 ions cm.sup.-3, roughly 30 times that found in neodymium-doped yttrium aluminum garnet (Nd:YAG) containing 1% neodymium. The high density of laser active sites in the stoichiometric neodymium phosphates causes laser pump radiation to be absorbed over short distances, typically 100 micrometers. The potential for minilaser applications in such fields as optical communications, handheld laser ranging devices, target designators, and the like is obvious.
The above compounds make use of particular crystalline lattice constraints which isolate neodymium ions to prevent self quenching at the high ion concentrations inherently present. The phosphate materials isolate Nd ions by typical distances of 0.5-0.7 nanometers by forming polymer chains in which the neodymium ions are separated by --O--P--O-- linkages.
Single crystal laser media are often difficult and costly to produce. The success achieved with single crystal laser materials has, however, spurred efforts to produce glass laser media which contain high concentrations of laser active ions such as neodymium. The ease of fabrication and lower cost of glass laser materials make them attractive choices over single crystals for minilaser applications.
The search for an adequate host glass composition for high concentration neodymium laser media is complicated, however, by the number and variety of constraints which are placed upon the selection of a suitable glass host. In addition to possessing the desirable properties of chemical stability and resistance to attack by moisture and corrosive agents, the glass should be capable of incorporating high concentrations of neodymium ion without devitrification or substantial changes in its desirable physical properties. Moreover, the glass host should possess properties which minimize emission line broadening of the laser radiation and non-radiative energy losses. These problems arise as the inter-site separation of laser active sites decreases with increasing concentrations of lasing ion. Phosphate glasses meet many of these criteria, but also frequently suffer from problems characteristic of phosphate materials. For example, due to strong hydrogen bonding between phosphate and water, it is often difficult to exclude water from glass compositions containing large amounts of phosphate. In turn, the finished glass materials are often hygroscopic, or easily degraded by moist or slightly acidic environments.
Laser active glass compositions approaching the formulations of the metaphosphate stoichiometric compounds have been prepared as described by Y. K. Voron'ko et al. in Sov. Phys. Dokl., 21, 146 (1976). These materials are somewhat limited in their usefulness as laser media, however, by emission line broadening and non-radiative energy losses which were attributed to the incorporation of water into the glass during fabrication. Techniques which have been employed to minimize water content in phosphate laser glasses during their fabrication include prolonged founding of the glass, vacuum founding, bubbling gases through the glass melt, and the incorporation of halide getters in the glass formulation. The minimum water content obtainable using these techniques is typically less than 0.2% as indicated by an infrared absorption coefficient of 6-8 cm.sup.-1 at a wavelength of 3.45 .mu.m.
A number of neodymium-containing phosphate glasses have been prepared in which the chemical stability and moisture resistance have been improved by incorporating an alkali or alkaline earth oxide into the glass. U.S. Pat. No. 3,979,322 issued to N. E. Alexeev et al. discloses a phosphate laser glass comprising phosphorus pentoxide, a laser active rare earth oxide, and oxides of the alkali or alkaline earth metals together with oxides of certain of the trivalent metals. As taught therein, the ratio of mono-, bi-, and trivalent metal oxides in total to phosphorus pentoxide exceeds unity.
According to the work of Deutschbein et al. in Rev. de Phys. App., 2:29 (1967), the incorporation of small polyvalent cations in a phosphate laser glass medium tends to broaden the emission band of the laser radiation.
It is, therefore, an object of this invention to provide a high neodymium content high phosphate glass which is useful as an active laser medium.
It is a further object of this invention to provide a high phosphate laser glass which can be easily fabricated by methods which do not require special care to exclude water during glass fabrication.
It is still a further object of this invention to provide a high neodymium content aluminophosphate glass which is chemically stable and resistant to moisture.
It is a still further object of this invention to provide a high neodymium content high phosphate glass in which increased chemical stability and resistance to moisture is achieved without the incorporation therein of an alkali or alkaline earth metal oxide.
It is still a further object of this invention to provide a laser which can be miniaturized by employing as the active laser medium a high phosphate glass which contains concentrations of laser active ions which approach the concentrations found in single crystal stoichiometric laser compounds.